In recent years, the need has increased for a high-performance, energy-saving color display that has high brightness, high resolution and low power consumption. The key to developing such a display is a high-performance fluorescent material that emits light of various colors, such as the three primary colors red, green and blue (RGB), with high brightness and fluorescence quantum yield. Such a high-performance fluorescent material is also essential in new energy-saving solid state lighting systems, the demand for which is increasing.
Fluorescent materials using rare earth ions and/or transition metal ions have been used for displays, etc. because of their excellent durability compared to organic dyes, etc. However, such fluorescent materials are not always satisfactory in terms of brightness and color-rendering properties. Therefore, there is a desire for fluorescent materials that have higher brightness levels than those of known fluorescent materials.
Semiconductor nanoparticles that can emit bright fluorescent light of various wavelengths according to particle size, even under UV radiation of the same wavelength, have been attracting attention as highly potential candidates for such a new high-performance fluorescent material. Fluorescent materials find application not only in displays and lighting systems but also as fluorescent probes that are produced by bonding such fluorescent materials to biomolecules. Even in such an application field, semiconductor nanoparticles whose fluorescence intensity reduction with time is small, compared to that of the organic dyes used so far, are attracting attention.
Semiconductors used as such fluorescent materials are mainly II-VI semiconductors (e.g., cadmium sulfide (CdS), zinc selenide (ZnSe), cadmium selenide (CdSe), zinc telluride (ZnTe), cadmium telluride (CdTe), etc.). Such semiconductors are direct-transition-type semiconductors and have an emission lifetime of about 10 nanoseconds. This emission lifetime is about 5 orders of magnitude shorter than known fluorescent materials using rare earth ions and/or transition metal ions whose transitions are mainly forbidden transitions. Therefore, such II-VI semiconductors can repeat the absorption of excitation light and emission of fluorescent light remarkably quickly, and fluorescent light of much higher brightness can be obtained. The short emission lifetime means a quick response to the ON and OFF of excitation light. Moreover, the deterioration of the semiconductor nanoparticles is much less than that of organic dyes.
Methods for producing such semiconductor nanoparticles in aqueous solutions and those in non-aqueous solutions have been developed. However, the semiconductor nanoparticles produced in a solution gradually aggregate, which starts immediately after the production, thus resulting in the deterioration of light emission characteristics. The nanoparticles produced in a non-aqueous solution have particularly poor resistance to water, and the fluorescence is rapidly reduced in the presence of even a trace amount of water. Another problem is that nanoparticle solutions, as is, are not suitable for technological applications. Therefore, it is necessary to incorporate, i.e., to disperse and fix semiconductor nanoparticles in a matrix such as transparent glass or the like, thus producing a solid-state material that maintains high-brightness light emission characteristics in various environments for a long period of time, and is suitable for technological application.
Examples of solid matrices in which nanoparticles are retained include glass and transparent organic polymer materials. Glass has the following advantages and is thus excellent: compared to organic polymers, glass has high transparency, high UV resistance, and low permeability to water and oxygen, and can therefore prevent the deterioration of nanoparticles dispersed in the matrix due to chemical changes for a long period of time. Using a sol-gel process to produce a glass is advantageous. When using a sol-gel process, a glass is formed under mild conditions, i.e., at room temperature and under normal pressure. Therefore, the semiconductor nanoparticles are dispersed and fixed in a transparent glass, while the particle size and high fluorescence quantum yield achieved immediately after the production thereof by an aqueous solution method are maintained. Once a solid glass incorporating nanoparticles is formed, the aggregation of nanoparticles and degradation by oxidation are less likely to progress, and a material that can stably emit high-brightness fluorescent light for a long period of time can be provided.
Examples of fluorescent glass produced by dispersing and fixing semiconductor nanoparticles in a glass using a sol-gel process include bulky glass, small glass particles, and thin glass films, and several production methods thereof have been attempted. Among these, small glass particles dispersed semiconductor nanoparticles therein are important as a powdery fluorescent material that is deposited on a substrate to produce a light-emitting device, such as a display or a lighting system, and as a fluorescent probe that is produced by bonding such small glass particles to biomolecules. The description below is limited to fluorescent small glass particles produced by dispersing and fixing semiconductor nanoparticles in a glass using a sol-gel process.
There are several known fluorescent small glass particles produced by dispersing and fixing semiconductor nanoparticles therein using a sol-gel process, and production methods thereof, as described below.
The first known small glass particles incorporating semiconductor nanoparticles are fluorescent small glass particles produced by dispersing and fixing semiconductor nanoparticles in a glass using a sol-gel process, and the fluorescence quantum yield thereof is about 1 to about 20%. The first production method thereof comprises: forming reverse micelles of a surfactant in a hydrophobic organic solvent; then adding an nanoparticle-dispersed aqueous solution to the reverse micelle solution to form reverse micelles containing the nanoparticle-dispersed aqueous solution therein; and adding to the reaction solution an alkoxide such as tetraethoxysilane (TEOS) as a reactant for glass formation to allow a sol-gel reaction to proceed in the reverse micelles, thus providing a small glass particle containing nanoparticles dispersed and fixed therein (Patent Document 1 to Non-Patent Documents 1 to 5).
The first production method was expected to prevent the aggregation of nanoparticles in the glass formation process by a sol-gel reaction, because the nanoparticles are separately present in the respective reverse micelles. This method was actually able to produce small glass particles containing nanoparticles dispersed and fixed therein.
However, the fluorescence quantum yield of nanoparticle-dispersed small glass particles obtained by the first production method is 5 to 10% in Patent Document 1 and Non-Patent Document 1; 14 to 20% in Non-Patent Document 2; 7% in Non-Patent Document 3; and 1 to 11% in Non-Patent Document 4. In any case, the fluorescence quantum yield is as low as 20% or less, and unsatisfactory for practical use.
Among the first production methods, those disclosed in Patent Document 1 and Non-Patent Document 1 produce small glass particles comprising semiconductor nanoparticles fixed near the outer surfaces of the glass particles, not inside the glass particles. When semiconductor nanoparticles are fixed near the outer surfaces of the glass particles, shielding of the semiconductor nanoparticle from the external atmosphere is insufficient, compared to semiconductor nanoparticles fixed inside the glass. Therefore, long-term stability of fluorescence characteristics tends to be insufficient. One reason considered as to why the semiconductor nanoparticles tend to be present near the outer surface of the glass particle is as follows: in the sol-gel reaction, alkoxide is hydrolyzed to form a silica network structure. When the hydrolysis of alkoxide is allowed to proceed, a mixture of an unreacted alkoxide having a low viscosity and semiconductor nanoparticles gradually changes to a mixture of a highly viscous gel and semiconductor nanoparticles. During that process, semiconductor nanoparticles are easily expelled from the silica network structure. As a result, after the sol-gel reaction has been completed to form a glass, the semiconductor nanoparticles are fixed near the outer surface of the formed glass particle, rather than inside the glass particle.
Bawendi, et al. (Non-Patent Document 6) describes fluorescent small glass particles containing semiconductor nanoparticles dispersed and fixed therein, and a production method thereof, referred to herein as a second production method. The second production method comprises: preparing nanoparticles in an organic solvent beforehand; dispersing the thus obtained nanoparticles into a solution containing an amino-group containing alcohol and a silane coupling agent to substitute organic molecules, such as thioglycolic acid, formed as a coating on the surface of nanoparticles in the nanoparticle preparation process with the amino group-containing alcohol and silane coupling agent; then adding the nanoparticle dispersion to a dispersion of nanoparticle-free small silica glass particles and an organic polymer in an alcohol; and then adding aqueous ammonia and alkoxide to allow a sol-gel reaction to proceed.
This method produces a fluorescent small glass particle comprising a nanoparticle-free small glass particle whose surface is coated with a nanoparticle-containing sol-gel glass layer. However, because the nanoparticles are present only near the surface layer of the glass particle and the core of the glass particle does not contain nanoparticles, it is impossible to increase the concentration of nanoparticles dispersed in the glass particle, and it is thus difficult to obtain strong luminescence and high fluorescence quantum yield.
Non-Patent Documents 7 and 8 describe a method of producing semiconductor-nanoparticle-containing small glass particles comprising: preparing semiconductor nanoparticles coated with a surfactant; chemically modifying the semiconductor nanoparticles with a silane coupling agent, alkoxide, or the like; and then hydrolyzing the silane coupling agent, alkoxide or the like, which is referred to herein as a third production method. However, this method has the following disadvantage for practical use. Because only one semiconductor nanoparticle can be contained in one small glass particle, the concentration of nanoparticles in the glass particles is very low, and it is difficult to obtain bright fluorescence.    Patent Document 1: Japanese Unexamined Patent Publication No. 2002-211935,    Non-Patent Document 1: Selvan, Li, Ando, Murase, Chemistry Letters, vol. 33, No. 4, page 434 (2004),    Non-Patent Document 2: Selvan, Tan, Ying, Advanced Materials, vol. 17, page 1620 (2005),    Non-Patent Document 3: Yang, Gao, Advanced Materials, vol. 17, page 2354 (2005),    Non-Patent Document 4: Yi, Selvan, Lee, Papaefthymiou, Kundaliya, Ying, Journal of American Chemical Society, vol. 127, page 4990 (2005),    Non-Patent Document 5: Darbandi, Thomann, Nann, Chemistry of Materials, vol. 17, page 5720 (2005),    Non-Patent Document 6: Chan, Zimmer, Stroh, Steckel, Jain, Bawendi, Advanced Materials, vol. 16, page 2092 (2004),    Non-Patent Document 7: Gerion, Pinaud, Williams, Parak, Zanchet, Weiss, Alivisatos, Journal of Physical Chemistry B, vol. 105, page 8861 (2001),    Non-Patent Document 8: Nann, Mulvaney, Angewandte Chemie International Edition, vol. 43, page 5393 (2004).